An exposure apparatus is one type of precision assembly that is commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, an optical assembly, a wafer stage assembly that retains a semiconductor wafer, a measurement system, and a control system. The resist coated wafer is placed in the path of the radiation emanating from a patterned mask and exposed by the radiation. When the resist is developed, the mask pattern is transferred onto the wafer. In microscopy, extreme ultra violet (EUV) radiation is transmitted through a thin specimen to a resist covered plate. When the resist is developed, a topographic shape relating to the specimen structure is left.
Immersion lithography is a technique which can enhance the resolution of projection lithography by permitting exposures with numerical aperture (NA) greater than one, which is the theoretical maximum for conventional “dry” systems. By filling the space between the final optical element and the resist-coated target (i.e., wafer), immersion lithography permits exposure with light that would otherwise be totally internally reflected at an optic-air interface. Numerical apertures as high as the index of the immersion liquid (or of the resist or lens material, whichever is least) are possible. Liquid immersion also increases the wafer depth of focus, i.e., the tolerable error in the vertical position of the wafer, by the index of the immersion liquid compared to a dry system with the same numerical aperture.
Immersion lithography thus has the potential to provide resolution enhancement equivalent to the shift from 248 to 193 nm. Unlike a shift in the exposure wavelength, however, the adoption of immersion would not require the development of new light sources, optical materials, or coatings, and should allow the use of the same or similar resists as conventional lithography at the same wavelength. In an immersion system where the final optical element and the wafer (and perhaps the stage as well) are in contact with the immersion fluid, much of the technology and design developed for conventional tools in areas such as contamination control, carry over directly to immersion lithography.
One of the challenges of immersion lithography is to design a system for delivery and recovery of an immersion fluid, such as water, between the final optical element and the wafer, so as to provide a stable condition for immersion lithography.
For example, injecting immersion fluid under an optical element that is inconsistent or non-uniform throughout the immersion area can adversely affect the lithography process. In addition, as immersion fluid moves in and out of the immersion area, air can be trapped under the optical element that can also affect the lithography process. Furthermore, residue immersion fluid left over in the immersion area from a previous immersion process can raise the temperature of the immersion fluid under the optical element. That is, immersion fluid left over from a previous process, which has been exposed to light again, can raise the temperature of the immersion fluid in a subsequent process under the optical element. This can adversely affect the wafer and lithography process. Therefore, what is needed is improved immersion lithography techniques for the flow and removal of immersion fluid.